1. Field of Invention
The present invention relates to phase-change type, reversible optical information-recording medium, being possible for recording, reproducing, erasing, and rewriting of information, by use of a laser beam.
2. Description of the Prior Arts
It is well known that some kinds of Te-based alloy film produce comparatively easily reversible phase transition by irradiation of a laser beam. Since, among them, the composition rich in Te-component makes it possible to obtain an amorphous state by relatively low power of laser, the application to recording medium has been so far tried.
For example, S. R. Ovsinsky et al. have first disclosed in U.S. Pat. No. 3,530,441 that such thin films as Te.sub.85 Ge.sub.15 and Te.sub.81 Ge.sub.15 S.sub.2 Sb.sub.2 produce a reversible phase-transition according to light with high density energy such as a laser beam. A. W. Smith has also disclosed a film of Te.sub.92 Ge.sub.2 As.sub.5 as a typical composition, and he has clarified that it could make recording (amorphization) and erasing (crystallization) runs of about 10.sup.4 times, and erasing (Applied Physics Letters, 18 (1971) p. 254). Moreover M. Chen et al. have disclosed that a film of Te.sub.87 Ge.sub.8 Sn.sub.5 as a typical composition could make repeating runs of more than 10.sup.6 times in a static test (Applied Physics Letters, 46 (1985) p. 734). Generally, in the recording medium utilizing the phase change between amorphous state and crystalline state, the procedure of the transition from crystal to amorphous state is used to recording process, attaching importance to recording speed. Since the above-mentioned compositions utilize those near eutectic mixture of Te and additives, they have lower melting points of about 400.degree. C. and can be easily amorphized. On the other hand, in erasing run, the procedure from amorphous state to crystal is used; since there exist many free Te-chain-structures inside each above-mentioned composition, progress of crystallization is obstructed by them. Therefore, as the chains at random direction remain inside even after solidification, it is necessary to keep the amorphous state for sufficiently long time and at high temperature (near Tg) in order to arrange the orientation, namely obtain the perfect crystalline state. There was a problem that the composition rich in Te could not in principle increase the erasing speed.
The existence of free Te reduces long-term thermal stability, particularly after repeating of recording/erasing run, and moreover it becomes a cause to restrict the cycling number itself. Though, it is assumed that in a thin film at "as deposited" state, Te atoms are uniformly dispersed with other atoms in a film, there occurs, during the repeating runs of recording/erasing, the separation between a part very rich in Te (relatively lower melting point) and a part rich in stable compound of Te and the additives (relatively higher melting point). Therefore, there occurs the phase separation and then the recording characteristics change. In this case, the composition with extremely abundant Te is very unstable as amorphous state with low crystallization temperature, and also gives a result to reduce the thermal stability. This phenomenon is assumed that it is mainly due to the fact that the composition near eutectic point could not exist as the stable compound and then it would transfer to more stable state in terms of internal energy. In the case of the extreme difference of characteristics, especially melting point, between Te and stoichiometric Te compound with additives produced by crystallization, the phase segregation proceeds more quickly, and it is difficult for separated phase to return again to the original uniform state. As a result, the recording and erasing could not be performed. As a way to solve the problem, it is necessary to increase the laser power, keep the film at a temperature above melting points of Te and Te compound for a long time, and quench after mixing them well again; by such treatments meaning of increasing sensitivity of recording by using excessive Te has come to be in vain. It has been impossible to realize the above treatments actually in dynamic system. Namely, in the composition excessive in Te, it could not be realized to obtain the satisfied characteristics such as the rewriting speed, thermal stability, and cyclability.